So I was unhappy with GoPro mounting options such as a gun mount (due to complications with loading, firing, too much movement, banging into rocks when hole hunting, etc.) or the head mount (drag during duck dives, getting caught on kelp, too much movement), so I decided to try this neoprene shoulder mount thing.
I think this thing is a winner. I mounted it slightly angled up so that when swimming and diving it captures where my center of mass is pointed, but slightly up to match my field of vision when swimming and diving.
I took it out today in pretty strong surge and current, with a good deal of kelp to boot, and it was rock solid. I also don't remember it getting caught in kelp even once, which surprised me.
Sometimes you try putting mounting something somewhere new thinking it would be less obtrusive and afterwards realize how much that spot comes into contact with stuff, and you just never realized it because you had nothing there before. Well in this case, it turned out to be a great spot and it never got stuck on anything even once.
Another nice thing on the shoulder mount is the camera doesn't move too much, which can be the case on a gun or head mount. I'm sure I lost some maneuverability and my hole shots probably won't come out as well, but on the whole my footage is much more stable and far less erratic than before.
So the shoulder placement was a clear winner in my opinion. The specific brand I got was a neoprene one from NeoPine, which I liked because of the width of the straps, which I figures would give some resistance to torsion, which it appeared to do as well. The Velcro was very strong and something very bad would have to happen to make the thing come off accidentally, but other companies' products may work very well too.
In summary:
Pros:
1) Highly secure
2) Very stable video
3) Unobtrusive and streamlined placement on body
4) Very little drag when diving
Cons:
1) Less maneuverability of shot angle
2) Positioning it so it doesn't capture too much of the side of your head can be a bit tricky